Semi-automatic and fully-automatic firearms may be designed to automatically expel the cartridge of a fired round of ammunition and chamber a new round of ammunition without the intervention of an operator. Such self-feeding of ammunition to a firearm is used to provide high rates of fire through both semi-automatic and fully-automatic firearms. The self-feeding operation may be performed by the internal operating group of the firearm using a variety of mechanism. A firearm may use the expanding gas from the fired round and/or the recoil from the fired round to provide energy to feed a new round of ammunition into the firearm.
In a closed-bolt firearm, a cycle of the firearm may include moving a firing pin to detonate a round in a chamber, retracting a bolt from the chamber, removing the casing of the fired round from the chamber, ejecting the casing of the fired round from the firearm, advancing a new round into alignment with the chamber, chambering the new round and closing securing the bolt to chamber the round. In an open-bolt firearm, a cycle of the firearm may include moving a bolt and firing pin forward, striking a round of ammunition, moving the bolt away from the chamber, removing the casing of the fired round from the chamber, ejecting the casing of the fired round from the firearm, and advancing a new round into alignment with the chamber. In both closed-bolt and open-bolt firearms, a cycle of the firearm includes advancing the next round into alignment with the chamber.
Each subsequent round may be moved into alignment with the chamber from a feed system. In some feed systems, each subsequent round is advanced by a mechanism independent of a cycle of the firearm. For example, a round may be moved from a storage position into alignment with the chamber by a mechanism in the ammunition storage device, such as in a magazine. In a magazine, multiple rounds of ammunition are stored with a follower at one end of the magazine. The follower may be a spring-loaded follower, or may include another mechanism to urge the rounds towards the chamber. A firearm using a magazine may passively have each subsequent round provided to the firearm such as in a semi-automatic handgun.
In other feed systems, each subsequent round is moved into alignment with the chamber by an operation of the firearm during each cycle. For example, a firearm including a clip may move each subsequent round from the clip to alignment with the chamber without an advancement mechanism in the clip. The entire clip and associated ammunition may move relative to a body of the firearm via an actuation mechanism of the firearm. The energy to move each round of the ammunition and/or move the clip may be provided by the firing of the gun, such as in a gas operated or gas-piston operated firearm, or by other sources, such as an electric motor. In a firearm that advances each subsequent round of ammunition by harnessing some of the energy of the firing of the gun, the energy may be harnessed by a gas-piston operating group, a gas impingement operating group, or a similar type of gas pressure activated operating group. The gas pressure activated operating group may convert at least some of the energy of an expanding gas within the firearm to a linear force to cycle the firearm.
Different firearms and different ammunitions may produce different gas pressures within the firearm when a round of ammunition is fired. The expanding gas forces a bullet or group of pellets down the barrel as the gas expands. The gas pressure is greatest at the start of the expansion and least near the terminal end of the barrel before the expanding gas is unconstrained and expands freely outside the barrel.